Fix Performance/Count cop error on empty selector block

Something like

```ruby
collection.reject { }.size
```

would lead to an error

Author: viralpraxis

changelog/fix_performance_count_cop_error_on_empty_selector_block.md

@@ -0,0 +1 @@
+* [#497](https://github.com/rubocop/rubocop-performance/pull/497): Fix `Performance/Count` cop error on empty selector block. ([@viralpraxis][])

lib/rubocop/cop/performance/count.rb

@@ -87,7 +87,9 @@ def autocorrect(corrector, node, selector_node, selector)
         end
 
         def eligible_node?(node)
-          !(node.parent && node.parent.block_type?)
+          return false if node.parent&.block_type?
+
+          node.receiver.call_type? || node.receiver.body
         end
 
         def source_starting_at(node)

spec/rubocop/cop/performance/count_spec.rb

@@ -353,4 +353,12 @@ def count(&block)
       RUBY
     end
   end
+
+  context 'with `reject` with empty block body' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        array.reject {}.size
+      RUBY
+    end
+  end
 end